Tokyo Electric Power Co said today its officials were unaware of a company emergency manual that defined a meltdown as damage exceeding 5 per cent of a reactor's fuel.
Instead, TEPCO described the condition of the reactors as less serious "core damage" for two months after the plant was hit by a massive earthquake and tsunami in March 2011, despite damage estimated to range from 25 to 55 per cent.
TEPCO said its initial wording may have been misleading, but didn't affect its emergency response.